东秦岭松树沟超镁铁岩侵位机制及其构造演化

 东秦岭松树沟蛇绿岩主要由镁铁质-超镁铁质岩石组成。镁铁质岩类的Sm-Nd全岩等时年龄为1030±46(2δ)Ma,ε_Nd(t)=+5.7±0.2,代表了蛇绿岩的形成时代。超镁铁质岩石由不同成因的橄榄质糜棱岩和中粗粒橄榄岩组成,橄榄质糜棱岩是地幔橄榄岩经历复杂变形并多次部分熔融的残余体,具LREE亏损特征,其中发育橄榄石高温位错构造和高温组构以及低温位错构造和低温组构。中粗粒橄榄岩具LREE略富集的分布特征,是地幔橄榄岩残余体再次部分熔融熔体分离结晶的产物。野外地质、地球化学、构造变形特征均表明超镁铁岩块是因洋壳俯冲而底辟侵位于上覆玄武岩中的地幔橄榄岩残余体。综合分析认为,松树沟蛇绿岩经历了古陆块裂解或洋脊扩张(1271-1440Ma)-洋壳形成(1030-1271Ma)-洋壳俯冲消减-橄榄岩块底辟侵位(983Ma)-蛇绿岩构造侵位及其后构造变形叠加改造的复杂演化过程。

THE EMPLACEMENT MECHANISM AND TECTONIC EVOLUTION OF ULTRAMAFITES IN SONGSHUGOU AREA, EASTERN QINLING

Songshugou ophiolites are mainly composed of mafic rocks and ultramafic rocks.The mafic rocks show simillar geochemical characteristics of N-MORB,T-MORB or E-MORB that suggest they were derived from evolved unequal mantle.The whole rock isochron age of the mafic rocks is 1030±46(2δ)Ma,ε_Nd(t)=+5.7±0.2,which represents the age of Songshugou ophiolites.The ultramafic rocks were osed of different genesis peridotitic mylonites and meso-macrograined peridotites.Theperidotitic mylonites are remains of mantle peridotites which underwent several phases of deformation and partial melting,and are characterized by depleted LREE.The high temperature and low temperature dislocations and fabrics of olivines were formed in peridotitic mylonites.The solid rheological conditions of high temperature deformati on areT>1000℃,δ₁-δ₃=113MPa,ε=1.307×10^-9/S,η=2.88×10¹⁷Pa/s and the slip system is (010)100].The solid rheological conditions of low temperature deformation are T=700℃,δ₁-δ₃=151.3Mpa,ε=1.17×10^-15/s,η=4.31×10²³Pa/s and the slip system is (010)001]or(100)001].The meso-macrograined peridotites are characterized by slightly enriched LREE.They were formed by the fractional crystallization of the melt that the mantle peridotitic remains partially remelted.The characteristics of geology,geochemistry and deformation suggest that the ultramafic block is mantle remains which was diapired into the overlying basalts because of the subduction of oceanic crust,and also suggest that Songshugou ophiolites underwent a complex evolutionary process which was the splitting of palaeo-continental block or the spreading(1271-1440Ma)-the formation of oceanic crust(1030-1271 Ma)-the subduction of oceanic crust-the diapiric emplacement of peridotitic block (983Ma)-tectonic emplacement of ophiolites and superimposed by the deformation in late stage.